Heresy in Christian usage is when a person or church denies or distorts scriptural doctrines which are essential for a proper understanding of Christianity. Heresy is not just acceptance of odd ideas or even aberrant beliefs. Heresy involves changing the nature of the Christian message. We can argue among ourselves about some things, but we can NEVER accept heresy into the church if we wish to remain authentically Christian.
We have to be careful about throwing around the “heresy label” and to do so, we have to be sure what is the authentically Christian message. A good discussion of this is here. There are a few doctrines on which there can be absolutely no compromise. They are false and it is important to know that they are false.
All of the classical heresies presented a different gospel from orthodox Christianity and should be rejected because they are inconsistent with revealed scripture. What may surprise many is that most, or even all, of the classical heresies are still in evidence in the modern world among professed Christians.
One of the most dangerous and powerful of the classical Christian heresies and one which is very evident today in the teachings of many “progressive” Christians is Arianism. This doctrine for a while threatened to gain ascendancy in the Roman Catholic Church but was condemned by the First Council of Nicaea in 324 and again at the First Council of Constantinople (Istanbul, Turkey) in 381. Protestants would say Arianism was rightly condemned because it is at total variance with scripture and changes the essential nature of the gospel. It is a totally different gospel. (2 Corinthians 11:4, Galatians 1:6-9)
Arius (born about 250 in Libya - died 336 in Constantinople) was a priest in Alexandria (al-Iskaderiyah), Egypt. He taught that Jesus was not pre-existent but was the first created being. Arius held to a monotheism which insisted that the godhead was unitary and could not be shared. The doctrine of the Trinity was rejected and Jesus, while still worthy of worship, was not God.
The teachings of Arius were rejected in 325 at the Council of Nicea (Iznik, Turkey) but were accepted in 357 at the Council of Sirmium (Sremska Mitrovica, Serbia). The issue went back and forth, sometimes violently. Constans, the Emperor of the West, was Nicene, while Constantius, the Emperor of the East, was an Arian. Emperor Valens (364-378) persecuted non-Arians in the East but under the teachings of the Cappadocian Fathers (Basil the Great, Gregory of Nyssa, and Gregory of Nazianzus) the Nicene definition began to gain ascendancy. Emperors Gratian (367-383) and Theodosiuis (379-395) were both Nicene and finally in 381, the Council of Constantinople denounced Arianism. The doctrine survived into the seventh century among several Germanic tribes such as the Vandals under Genseric.
Variations of Arianism survive among the Jehovah's Witnesses, the Mormons, the Unitarians, the Church of God (7th day) Salem Conference, and in some Branhamite sects (followers of the Pentecostal leader William M. Branham, 1909-1965).
Postmodern Progressive Christianity can be viewed as a modified form of Arianism. Man is the true measure of all things and Jesus's humanity is stressed to the near exclusion of his divinity. He is viewed as a brilliant moral teacher and a beautiful example of humanity at its highest. He was a human so full of God that He became "divine." He shows us the divinity within ourselves. This is just one baby step away from the neo-Hinduism of the modern New Age movement. It is certainly not orthodox Christianity.